(1) Field of the Invention
The present invention relates in general to image display devices, display panels, and driving methods thereof implemented within display driver circuits, and particularly to the drive circuitry of matrix large-screen and high resolution organic light-emitting diode (OLED) displays, especially circuits used in LED drivers manufactured as semiconductor integrated circuits. Even more particularly, this invention relates to a multi line address method saving calculations and processing power and optimizing performance as well.
(2) Description of the Prior Art
Electronic display devices are to date still from the LCD (Liquid Crystal Display) type fabricated in STN (Standard Twisted Nematic) or TFT (Thin Film Transistor) technology needing additional back-lighting but of late are more often also made as LED (Light Emitting Diode) displays in form of self-luminescent OLED (Organic LED) and PLED (Polymer LED also named as PolyLED) devices. Ever more such displays capable to exhibit their own luminosity without extra light sources are preferred. OLED technology incorporates organic luminescent materials that, when sandwiched between electrodes (anode, cathode) and subjected to a DC electric current, produce intense light of a variety of colors. Nearly the same is holding for PLED devices where polymer materials are used instead. Hence we will use in the following the terms OLED and PLED mostly in an exchangeable meaning. Similar capability can be achieved with Surface conduction Electron Emitter Displays (SEDs), High Dynamic Range (HDR) displays, Field Emission Displays (FED) and QDLED-Displays making use of Quantum Dot crystals. Currently OLED and PLED displays are commonly used, available in PMOLED (Passive Matrix) OLED and AMOLED (Active Matrix) OLED structure forms, differentiated by their driving methods and circuits. PMOLEDs are much simpler to manufacture than AMOLEDs because there is no TFT substrate for active components needed, and as a result fewer processing steps are required in the manufacturing line. OLED and PLED displays have several advantages compared to other display technologies: they are self-luminescing i.e. self-light-emitting not needing a backlight, do have high brightness and high luminance efficiency, exhibit short response times, a wide visual or viewing angle, a high contrast ratio, show a super-thin appearance (even flexible panel constructions are possible), they are power saving, offer a wide temperature tolerance, and so forth. OLEDs and PLEDs are also useful in a variety of applications as discrete light-emitting devices or as active elements for light-emitting arrays or displays, such as Flat-Panel Displays (FPD) of all kind and size. Depending on the types of substrates used for OLED and PLED manufacture, there are various types of implementations: Transparent OLEDs wherein transparent substrates for cathode and anode are used, which because of these transparent components can pass light in both directions and thus are especially useful in head-mounted display devices; Top-emitting OLEDs which use either opaque or reflective substrates, allowing light to be emitted in one direction only, and which are the most used types; Foldable OLEDs using highly flexible substrates, which help to reduce breakage of the display material thus allowing many new applications; and White OLEDs, used to emit white light which is brighter, more uniform and more energy efficient than other materials used for lighting.
The most obvious difference between PMOLED and AMOLED displays is the construction of their light-emitting elements or pixels. Such pixels of PMOLED devices are simply represented by the LED itself, created at the cross-overs of the dedicated conductive matrix wires. No additional storage or switching elements are included, only the capacitance of the LED itself determines the lighting dynamics. The PMOLED display device therefore requires a relatively high amount of power to operate in order to sustain a flicker free image. In addition, the display size of a PMOLED display device is limited by this matrix structure. The larger the matrix becomes the longer the wires get with an increase of their harmful properties, such as resitivity losses on wires and parasitic capacitances between lines responsible e.g. for perturbing crosstalk effects. Furthermore, as the number of conductive lines increases, the aperture ratio of PMOLED display devices decreases. In contrast, AMOLED display devices are highly efficient and can produce a high-quality image for a large display size with relatively low power. In general, in an AMOLED display device, a voltage controlling a current applied to the light-emitting element or pixel is stored in a storage capacitor. Accordingly, the voltage in the storage capacitor can be applied to the pixel until the next complete image content or frame is fed in and the pixel can continuously display the image during that one frame. As a result, an AMOLED device has a lower power consumption, with high resolution even at larger display sizes because it is able to display complete image frames with a constant brightness in spite of low driving currents, at the expense of additional switching elements for each pixel however, whereby the switch is usually implemented as Thin Film Transistor (TFT). PMOLED displays can therefore generally be manufactured at a lower cost than AMOLED displays.
Although the PMOLED display has a simple configuration and an advantage in terms of cost, the difficulty in realizing a display with large size and high brightness as well as low power consumption and high lifetime is limiting its use. Therefore, the AMOLED display in which the current flowing in a light-emitting element is controlled by one or more TFTs and the related voltage is stored by a capacitor where both components are disposed in each of the pixel circuits has its own preferred fields of application. An AMOLED display device has problems however in that variations in component characteristics of the pixel circuit may exist and thus brightness of each pixel which make up the display screen may vary.
As is well known for OLED and PLED displays, optimum performance especially with high-brightness LEDs is achieved only when the LEDs are driven by current sources rather than by voltage sources, the currents of which are delivered by individually controlled display drivers with highly precise current sources directly driving the LED pixels, whereby the LED element of each pixel itself is an electric component with diode characteristics including also parasitic resistances and capacitances.
In the PMOLED case no further components are needed to build the picture elements or pixels for the dots of the display matrix. A pixel, by definition, is therefore a single point or unit of an image, whereby its color is to be chosen, for color displays in a real-time programmable way. In monochrome displays a pixel only displays a single color whereby that color is not individually changeable, only for the display on the whole at production time. However in color displays, a pixel is capable to individually change its color and therefore has to include an arrangement of so-called sub-pixels, at least one sub-pixel for each of its elementary color components according to the color dispersion method chosen, as for both, PMOLED and AMOLED devices.
In the AMOLED case however the OLED sandwich structures are combined with electronic switches (transistors or diodes, especially Metal-Insulator-Metal (MIM) devices) and separate charge storing elements (capacitors) to form pixels that make up the dots of a modern matrix display. Dots for color displays therefore generally comprise more than one pixel, and thus are made up of sub-pixels emitting for example red, green, blue and of late also white light, which are individually controlled and driven. Various differently complex sub-pixel circuits have been developed making additional use of several TFT transistors and storage capacitors in order to overcome onerous side-effects of the intrinsic light emissive material of the pixels—such as degradation during lifetime, delayed response times for activation and deactivation of the pixel, and the like.
With reference to the more elaborate sub-pixel circuits for ON/OFF controlling the organic or polymeric light-emitting cell of each sub-pixel there are the already known two elementary driving methods for PMOLED and AMOLED displays, one is the Passive Matrix (PM) driving method and the other is the Active Matrix (AM) driving method using TFTs. In the PM driving method, anode and cathode electrodes are arranged perpendicular to each other to selectively drive the lines. On the other hand, in the AM driving method, TFTs and a charge storing capacitor are coupled to the pixel electrodes so as to sustain a voltage by the capacity of the capacitor. According to the form of the signals applied to the capacitor to sustain the voltage, the AM driving method can furthermore fundamentally be divided into a voltage programming mode and a current programming mode, whereby of late the current programming mode is preferred as already mentioned above. OLED displays are thus normally operated as current-controlled display devices. Nevertheless, for high-content displays realized as large matrix arrays, a multiplexing mode is also a necessity. In this context, though OLED devices are essentially current-controlled devices, a voltage drive mode is chosen for a short period before the current drive mode is established, which is operating as charge drive for the parasitic internal parallel capacitances of the OLED sub-pixel diodes. The electrical model of a sub-pixel of an OLED consists of a Light Emitting Diode (LED) and the parasitic capacitance modeled by a capacitor in parallel. A sub-pixel thus emits light when current passes through the diode. In a current driving system, the constant current source connects to the sub-pixel to turn it on. This charges up the capacitor linearly. Before the sub-pixel voltage reaches the diode's threshold or forward voltage, there is no current flowing through the diode and the sub-pixel is still OFF. Supply current is consumed only for charging the capacitor during this period. If the capacitance is large the sub-pixel is off for a long time. And it is ON only after the sub-pixel voltage has reached the threshold voltage level. These parasitic capacitances may become rather large depending on the size of the sub-pixel. The time of their charging-up until reaching the sub-pixel diode's threshold voltage is thus referred to as pre-charge period. Therefore, for a multiplexed matrix OLED display, both a current drive and a voltage drive are required. Because larger OLED displays exhibit these high capacitance characteristics, a normally substantial pre-charge current is injected voltage driven to bring the diode up to near its operating current prior to enabling the diode. Thus, time and power are not wasted for charging and discharging the relatively high capacitance that is inherent to large OLED display sub-pixels and the lifetime of the diodes are prolonged because the diodes are not required to swing the full voltage range during each cycle. Consequently having driven the OLED sub-pixel by pre-charging into the constant current driven and linear time function voltage rising region a Pulse Width Modulation (PWM) brightness control method for OLED sub-pixels is now feasible with high accuracy. The longer the constant current is applied, the brighter the OLED sub-pixel shines.
Generally the PM and AM OLED technology provide bright, vivid colors in high resolution and at wide viewing angles, additionally also exhibiting a high response speed in large but nevertheless slim FPD devices. OLED devices' technological advantages of high brightness and high luminance efficiency, short response time and wide visual angle, together with its power saving operation and wide temperature tolerance, have established unequaled features for large screens, offering high resolution displays with up to several million pixels and diagonal sizes up to 60 inches. However, OLED technology in very large-screen or huge-screen display applications is currently still on its way into the mass market; examples include huge time-table displays at train stations, in airports, or at harbors, or displays for large marketing advertisements and mass-public informational purposes including those displaying share prices in stock exchanges, and huge indoor or even outdoor stadium displays. OLED color displays are expected to offer substantial advantages compared to other technologies currently in common use: wide dynamic range of colors, high contrast, superior light intensity and lesser depending on various external environmental factors including ambient light, humidity, and temperature. For example, outdoor displays are required to produce more white color contrast under daylight conditions and during the night show more black color contrast. Accordingly, light output must be greater in bright sunlight and lower during darker, inclement weather conditions. The intensity of the light emission produced by an OLED pixel is directly proportional to the amount of current driving it. Therefore, the more light output needed, the more current has to be fed to the pixels which on the other hand is detrimental to the lifetime of the pixels.
FIG. 1 Prior Art now shows as circuit schematics the drawing for such an FPD, wherein a matrix display device converts electric signals processed by an information processing device into an image, visible on a FPD screen. Numerous circuits for complete FPDs exist as prior art in many variants, they shall be summarized here in form of an exemplary circuit shown as FIG. 1 Prior Art:
From FIG. 1 Prior Art can be recognized an Image Data Source 10 being connected via a bi-directional data bus and feeding its image data stream normally comprising a multitude of image frames into an Image Storage 15 unit, capable of storing multiple image frames, whereby every image frame contains in general successive image data from said incoming image data stream. That Image Storage 15 unit is again bi-directionally connected to a Display & Timing Controller 20 unit, comprising inter alia data and/or signal Logic circuits and data and/or signal processing units.
Display & Timing Controller 20 unit then prepares and conditions those image frame data as they come in from the Image Storage 15 unit, and delivers these data now in an appropriately transformed manner via image data and control signal bus systems 23, 24, 25, and 26 to the respective, closely display matrix adapted electronic driver units 30, 35, 40, and 45 of the FPD's literal Pixel Matrix 50. The Pixel Matrix 50 of the FPD includes a plurality of X-Lines 53 (i=1, 2 . . . m−1, m) extended along a first direction of an array substrate serving as material medium for the screen, a plurality of Y-Lines 54 (j=1, 2 . . . n−1, n) extended along a second direction of the array substrate that is substantially perpendicular to the first direction, and a plurality of sub-pixel 55 elements P(i, j) each electrically connected to one of the X-Lines and one of the Y-Lines. In this manner a Cartesian X-Y system of coordinates is established, mathematically spoken. From the terminology of mathematics here also the designations used are derived. Each i, j-indexed pair of X-Y coordinates thus uniquely identifies a sub-pixel element P(i, j) within 55. Many other terminologies in the context of FPD designations are in wide-spread use however, depending on the point of view (POV) taken in explaining the configuration. A possible interchange of the sequence X-Y into Y-X comes from the fact that the designation of the axes is freely interchangeable with its coordinated line designations, in case of a PM-structure these axes are even functionally interchangeable, because the construction of PMOLED pixels is fully symmetrical; besides polarity of the OLEDs only, where anode and cathode are interchanged which can easily be accounted for by inversely adapted voltage polarities however. Most closely related are the terms Rows and Columns for the Y/X- and X/Y-Lines respectively, which use directly the mathematical matrix designations for these parts. Using topological terminology leads to Horizontal and Vertical, which is mapped to Y/X- and X/Y-Lines again. From an FPD operational POV the Y- and X-Lines are called Scan-Lines and Data-Lines respectively, which is a rather often used terminology in fact. This operational POV sees an FPD as an image display device including Data-Lines for transmitting image data voltages representing the image signals, Scan-Lines for transmitting appropriate multiplex select signals scanning the matrix, with sub-pixel circuits for each image point coupled directly to those Data- and Scan-Lines. An even more technical aspect leads to the electrical POV valid however for AM-displays only, namely Gate-Lines and Source-Lines stemming from the utilized TFT-switching transistors in AM sub-pixel circuits. This electrical POV sees a Scan-Driver driving an AM-display device having a plurality of Gate-Lines transferring multiplex scan signals, and a Data-Driver driving a plurality of Source-Lines transferring image data signals. Also from the technical or electrical POV often in use for PM-displays are the terms Anode and Cathode, reminding directly of the OLED's diode function. Thus the designations Anode- and Cathode-Lines are used, as well as Anode- and Cathode-Drivers. All these designations are used in the case of the dynamic operation of multiplexed FPDs. There is however also a static, non-multiplexed operation possible, using fewer pixels only, then the Y/X- and X/Y-Lines are called Segment-Lines and Common-Lines or vice-versa, which becomes evident from the arrangements for simple displays, e.g. the commonly used 7-segment cipher displays. It is furthermore obvious that in case of a PMOLED display it is arbitrary which lines are labelled Row lines and which Column lines, Rows and Columns can be used interchangeably. From a methodological POV, the X/Y- and Y/X-terminology is avoided if not only strictly symmetrical issues are concerned, which is seldom the case, also for PM-arrays; the Row/Column or Scan/Data designations are easier to understand and remember, the Gate/Source naming is usable for AM-structures only, and even there it is not simply applicable any more because of the complex pixel-circuits with diodes as switching elements etc., the Anode/Cathode terminology is popular instead.
Consequently the display matrix adapted electronic driver units 30, 35, 40, and 45 bear the following names for unit 30: X/Y-Driver or Data-Driver or Source-Driver or Matrix-Column- or Horizontal-Drive-circuit which is driving the vertically running X/Y-Lines, Data-, Source-, or Column-Lines. Unit 40 correspondingly becomes designated as Y/X-Driver or Scan-Driver or Gate-Driver or Matrix-Row- or Vertical-Drive-circuit again now driving the horizontally running Y/X-Lines, Scan-, Gate-, or Row-Lines. Unit 35 is a possible coordinated driver circuit usually performing auxiliary functions such as pre-charge or discharge operations, compensation signal adding or secondary emit control functions for its attributed X/Y-Driver or Data-Driver or Source-Driver or Matrix-Column-circuit 30. In the same manner is unit 45 a possible coordinated driver circuit also performing auxiliary functions such as pre-charge or discharge operations, compensation signal adding or secondary emit control functions for its attributed Y/X-Driver or Scan-Driver or Gate-Driver or Matrix-Row-circuit 40. It shall especially be mentioned that all these functions may also be incorporated into the main driver circuits 30 and 40 as shown in FIG. 1 Prior Art for the corresponding pre-charge sections 31 and 41 respectively. From this can then be deduced that all the horizontally 54 and vertically 53 running data, select, scan, and control signals 53, 54 leading to their related sub-pixel circuits within 55 are possibly bundled in signal bus lines comprising multiple wires. Equally should be mentioned that the display matrix area may be separated into multiple sub-areas used for displaying only partial frames, so-called sub-frames, together with an appropriate adaptation of corresponding driver circuits and data and control signals. In order to be able to fulfill all the necessary tasks the mentioned display driver circuits or units 30, 35, 40, and 45 may contain needed sub components such as memory registers, shift-registers, switches, multiplexers, voltage level shifter circuits, programmable voltage and/or current sources and/or sinks, and additional clocks or timers. FIG. 1 Prior Art also unveils the existence of several power supplies 70 and 75 intended for generating and/or delivering various voltages and currents for being used as e.g. Row ON/OFF Voltage Source, Column ON/OFF Voltage Source, or as Column Compliance Voltage Source, or as Pre/Discharge Source, or the like. The generated voltages or currents are used for OLED pixel operations like applying the Pre-charge Pulse, setting Display Sub-pixel ON/OFF or accelerating the pixel OFF responses by injecting an extra Discharge Pulse. During a multiplexed image display operation the Scan/Row-Lines 54 are activated in sequence. When one of the Scan/Row-Lines 54 is activated, a data signal is applied to the selected sub-pixel elements in 55 through the Data/Column-Lines 53, so that the respective sub-pixel elements in 55 are electrically activated. When these selected sub-pixel elements in 55 are electrically activated, normally all additionally estimated necessary or needed auxiliary signals are appropriately synthesized defining the drive or data signals for all the sub-pixel elements in 55 thus correctly controlling the entire display pixel 55 made up of different sub-pixels. As a result, an optical activity is enabled to display the desired image. The time period during which first through last Scan/Row lines are activated is referred to as one frame, in the case only of regular single sequential scanning operations however.
With FIG. 2A Prior Art a more detailed view onto the Pixel Matrix 50 from FIG. 1 Prior Art of an FPD with PMOLED display matrix is depicted, schematizing the current driving functions of the Data/Column 30 drivers and the scanning operations of the Scan/Row drivers 40 by showing switched constant current sources as well as simple switches instead for each Column and Row respectively. The OLED pixels on the other hand are represented together with their parasitic elements total resistance R_tot and parallel capacitance C_p, just the same as the Row and Column wires with their loss resistances R_row and R_column. No other components are comprised in the passive matrix diode display array. As operating example is shown a state with Row R1 selected, i.e. its according switch is closed and Columns C1 and C2 are ON with all other Columns OFF, i.e. the related switches are closed only in the first two columns and thus only there currents can flow, and if the threshold voltages of the two switched ON diodes are surpassed after the parasitic capacitances of these two diodes are sufficiently charged-up, the OLED pixels (1,1) and (1,2) are shining bright. From this description and drawing the need for pre-charging OLED displays is easy to understand, if fast responses are required. The influence of all the other parasitic and lossy elements in PMOLED displays is also clearly illustrated.
Several different addressing schemes are used for individual addressing of the display pixels 55 of a display matrix, whereby in fact the addressing designates the selection or activation of single sub-pixels within a certain OLED pixel 55 dot. In general the individual sub-pixels in a matrix row are activated or selected by a Matrix Row signal formerly designated also as Scan/Row signals for a Row Select Time, whereas the image data to be displayed are supplied via individual Matrix Column or Data/Column signal lines. The most common addressing scheme formerly used mainly for LCD devices is the so called Alt & Pleshko driving scheme. Hereby each Matrix Row is activated separately. At the time the respective Matrix Row is selected or scanned the required image Data signals are applied to the Matrix Column via their Data/Column lines. So each display sub-pixel in the selected Matrix Row will show its programmed brightness as controlled by appropriate PWM Data signals as already explained farther above, which means each dot displays its correct color in case of color FPDs. After all dots within one Matrix Row have been completely activated, the next Matrix Row will be selected until all Matrix Rows of the display have been selected one time to display a complete image frame. Thereby, as already mentioned above a frame is defined as the time it takes to select all Matrix Rows of the FPD in case of Alt & Pleshko and thus driving every Matrix Row exactly once.
However, the PMOLED displays used in many modern applications encounter several common issues when the displays become larger and especially if they should also display video streams with moving pictures. These issues include sensible higher power consumption, thus an elevated operating temperature and, the larger they get a slower frame response with poorer contrast. A lifetime reduction becomes noticeable too, especially when PMOLED displays are frequently in use for displaying moving pictures at higher frame rates. In order to improve the quality of PMOLED displays, new driving methods using more evaluated addressing schemes are required. Simple addressing schemes did always address only single lines or rows of an FPD matrix at a time, applying ordinary multiplexing techniques. They are therefore subsumed and known as Single Line Addressing (SLA) techniques. More sophisticated addressing or driving schemes are the Multiple Line Addressing schemes (MLA), also known as Multiple Row Addressing (MRA). Groups of Matrix Lines or Rows are simultaneously driven and encoded image information is applied to Matrix Columns as Data/Column signals. The Data/Column voltage signal is now applied to the corresponding Matrix Column so that the corresponding OLED sub-pixels are all driven at the same time with image data.
FIG. 2B Prior Art shall now illustrate a simple MLA operation, looking back onto the already explained passive OLED pixel matrix from FIG. 2A Prior Art. Two lines are taken as example here, which are addressed together, namely Row Rj and Row Rj+1. The corresponding Scan/Row drivers are again replaced here by simple switches, which are closed if the lines are selected. The individual Data/Column drivers are represented however by controlled current sources, therefore no switches are needed here. As can be seen from the drawing, all the diodes within rows Rj and Rj+1 are connected to these controlled current sources and are therefore driven two in parallel in each column, that is in the example chosen here. From every Data/Column current source in MLA FPD driver circuits there is always the sum of currents drawn for all the OLEDs selected in multiple lines. Supplying all the diodes in the selected rows together at the same time thus always makes necessary exactly that same multiple of the current which would drive only one diode to the same brightness, the multiple according to the number of lines used in the MLA scheme. Additionally all the MLA schemes have to take into account the provisions to be made for needed pre-charging methods, which as a matter of course have also to be applied here, however considering the multiple demand of current just in the same way as for the Data/Column driving currents. What also can be understood easily from here, is the fact that all the OLEDs from each of the lines in an MLA scheme which are being driven simultaneously together can only receive identical contents, because of their identical Data/Column driving currents they receive.
Coming back now to PMOLED displays, where each Horizontal line is driven by a Row driving image Scan signal, and each Vertical line is driven by a Column driving image Data signal, it shall be recalled that the Row/Scan driver applies its Scan signal to one selected horizontal Row; while the Column/Data driver normally supplies all its image Data signals to all the vertical Column lines at the same time in conformity with the image Data content of the line being selected by the Row/Scan signal. Each pixel connected to its vertical Column line of the display now has power applied to it and is therefore illuminated. This driving mode is also designated as Single Line Addressing (SLA) mode. Having large numbers of vertical Column lines results in large currents to charge many OLED pixels at once. Therefore sometimes the exact points in time for the image Data supplied by the Column/Data driver are slightly delayed against each other, that is evenly distributed over the time slot available for the activation of the pixels of the whole row in order not to overly load the power supplies for the drivers. In other words, to create an image for every Row the Row/Scan signal is maintained as each of the Column lines is activated in turn until the complete Row has been addressed, and then the next Row is selected and the process repeated until all Rows are completed. Preferably is, however, to allow individual pixels to remain on for a longer time and hence reduce the overall drive level. The conventional method of varying pixel brightness is to vary the ON-time of each pixel using Pulse Width Modulation (PWM). In a conventional PWM scheme a pixel is either fully ON or completely OFF but the apparent brightness of a pixel varies because of an over the time integration within the observer's eyes. An alternative method is to vary the pixel driving current. Thus absolute current value and duration of the current flow for the driving Column/Data pulse can be played off against each other. Therefore current level can be lower if time is longer to obtain the same brightness impression. Thus usually one Row is selected and all the Columns are written in parallel despite of the summed-up current load this creates, that is a current is driven onto each of the Column lines simultaneously to illuminate each pixel in a Row to its desired brightness. The method mentioned above, where each pixel in a Column is being addressed in turn before the next Column is addressed is not often used because, inter alia, of the effect of column capacitance.
As already mentioned farther above it is usual practice to provide a current-controlled rather than a voltage-controlled Column/Data drive signal to an OLED because the brightness of an OLED is determined by the current flowing through the device, thus determining the number of generated photons. In a PMOLED matrix configuration the brightness can vary across the area of a display and also with operation time, temperature, and age, making it difficult to predict how bright a pixel will appear when driven by a voltage. Especially in color displays the accuracy of color representations may be affected. Another major challenge with using PMOLED in high resolution displays is that the operating lifetime is limited, as already adumbrated. The reason for this is that the current must be injected into each pixel diode within one horizontal line period, during which each row is selected. This means that as the resolution is increased a higher current must be injected into the pixel OLED over a shorter period of time in order to achieve the desired brightness. This high current accelerates the aging process that occurs inside the OLED, reducing the intensity of its light output over time; AMOLED displays do not have this limitation to the same degree, since the current can be injected during the whole vertical image frame period.
Many different MLA schemes exist. Modern MLA schemes have been further expanded and continuously developed into the Consecutive MLA (CMLA) scheme, a rather complex matrix decomposition method combining MLA and Single Line Addressing (SLA) techniques described in the WIPO Patent Application (WO/2007/079947 to Xu et al.) cited below, which however not in all cases delivers optimal solutions, sometimes even augmenting the number of necessary charging operations. Total Matrix Addressing (TMA) from U.S. Patent Application (2008/0246703 to Smith et al.) also cited below, despite and eventually because of its need of substantial processing power, is also not very satisfactory concerning overall efficiency in terms of power saving.
As can already be seen from the above the goal to both get the benefits of MLA schemes for FPD driving and at the same time limit the processing power requirements in favor of a low power consumption and augmented life-time of the whole FPD product is not easy to attain, a multitude of MLA schemes have been proposed with varying success given the surplus expenses needed.
Even though focus in the parts above has been mainly on Passive Matrix (PMOLED) solutions, a comparison of Active and Passive Matrix technology shall be given in the following passages.
Active vs. Passive Matrix—Introduction and Overview:
Instead of the above-mentioned simple matrix or passive matrix electro-optical display only, an active matrix driving in which additional electrical components forming separate active pixel-circuits are provided for each pixel and sub-pixel elements has many advantages and offers many other possibilities for additional tasks.
In comparison to the passive matrix driving, a light emitting display device using the active matrix is driven by a pixel or sub-pixel circuit with at least one transistor and a storage capacitor (electric condenser) to maintain a voltage by the storage condenser capacitance which in turn controls the driving current for the light emitting element. Each light emitting device in a passive matrix type display apparatus emits light only at an instant when the light emitting device is selected, whereas a light emitting device in an active matrix type display apparatus continues emitting light even after completion of writing. Thus, the active matrix type display device is advantageous especially for use as a large high-definition display in that an active matrix type display device can decrease peak brightness and peak current of the light emitting element as compared with a passive matrix type display device.
An OLED as light emitting element in an electro-optical device has a low viewing angle dependency because of its self-luminous type, and does not need a backlight or a reflected light illumination. OLEDs constitute a particularly advantageous form of electro-optic display. They are bright, colorful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. OLEDs may be fabricated using either polymers or small molecules in a range of colors (or in multi-colored displays), depending upon the materials used. On this accounts, it has excellent characteristics as a display panel, such as its relatively low power consumption and the adaptability to being made thin. The OLED element e.g. is an electrical current type passive light emitting element in which a light-emitting state can not be maintained when the electrical current is cut off, because it does not have a voltage holding characteristic like a liquid crystal element. For this reason, when the light emitting element is driven in an active matrix method, a configuration is generally used in which a voltage corresponding to gray scale of a pixel is applied to a gate of a driving transistor, the voltage is hold by the gate capacitance, and a current corresponding to the gate voltage is input to the light emitting element by the driving transistor during a writing and driving period (selection period).
Important aspects of the active matrix structure and pixel element circuits and distinct facets of used principles and methods as well as problems and solutions concerning the many already existing active-matrix display solutions shall now be listed and described, or at least mentioned here for reference.
Active Matrix Structure:
A plurality of scan and data lines controlling the display matrix pixel and sub-pixel circuit and their operation plus a variety of additional specific control signals altogether powered by multiple different supply voltages and currents at various levels are used in conjunction with various known active-matrix display solutions. Usually a light emitting display device includes a data line, multiple signal lines, a pixel circuit, and a data driver for supplying a precharge current to the data line according to a control signal and supplying a data current to the data line according to another control signal.
Pixel Element Circuits:
Various internal components (preferably in thin-film technology) comprised in the pixel sub-circuits such as multiple transistors and/or diodes for switching and driving operations by voltage controlled current supply (e.g. with current mirrors) plus extra accumulation or storage capacitors adapted to their specific pixel or sub-pixel elements are implemented by different known active-matrix display solutions. To effectively reduce or prevent the deterioration of display quality caused by the errors caused by unwanted variations in the current supplied to an electro-optical element the pixel circuit is used.
Principles and Methods:
An active matrix method may include a voltage programming method or a current programming method, depending upon signal forms supplied for programming the voltage at a capacitor within each of the pixel sub-circuits. An important benefit of the conventional current programming type pixel circuit is that the current which flows to the OLED has a substantially uniform characteristic over the whole panel, compared to the voltage programming type pixel circuit.
In order to precisely control brightness in an electro-optical device, the amount of power supplied to the light emitting elements must be precisely controlled. In particular, because OLED light emitting elements are current-driven type electro-optical elements, the brightness is directly affected by the current. Hence there is a need to precisely supply the desired current to organic light emitting elements; and in addition to optimizing the driving circuit and driving method, the pixel layout must also be optimized. Problems encountered in actual pixel layouts include, for example, contact between pixel electrodes and peripheral circuitry, stability of holding capacitances, and transistor turn-off currents.
The display driver circuitry for electro-optic displays may include a driver to drive a light emitting element in accordance with a drive voltage, but also a photosensitive device optically coupled to the light emitting display element to pass a current dependent upon illumination reaching the photosensitive device and thus controlling the driver according to the environmental lighting. The circuit can then be operated in a number of different modes and provides flexible control of an electro-optical display element such as an OLED pixel.
The control circuit which includes a charge storage capacitor and a photosensitive device coupled to the storage capacitor for regulating charge stored on the storage capacitor in accordance with light falling on the photosensitive device may further comprise means for the independent voltage control of a gate terminal of the photosensitive device, preferably a phototransistor. In this way a more efficient and flexible biasing of the phototransistor is possible. In addition, it becomes possible to use the phototransistor as a TFT switch. This dual function (phototransistor/TFT switch) enables the pixel circuit to provide additional features; for example duty-cycle techniques for motion blur compensation. The inclusion of the photosensitive device improves the uniformity of the display and compensates for aging effects of the display device. The photosensitive device is a phototransistor having a gate terminal, which is attached to the anode of the light emitting display element, an OLED e.g. The phototransistor is not driven to be conductive and therefore acts as a photodiode responding almost linearly to the incident light.
Duty-cycle techniques for motion compensation will be applied especially for moving images. By switching the phototransistor on, the gate voltage of the driving transistor is set to the power line voltage. This turns the drive TFT off, and no current flows through the PLED. In this way the light output can be prematurely stopped. In still images this is not required.
In a light emitting device performing image display additionally separate non-volatile memory circuits may be contained in every one pixel. Thus the electro-optical device has a function for storing frame portions of the digital image signal in the volatile memory circuits. By performing display of a static image in accordance with repeatedly reading out, for each frame, the digital image signal stored once in the memory circuits and performing display, drive of a source signal line driver circuit can be stopped during that period. Further, a digital image signal stored in the non-volatile memory circuits is stored even after a power source is cut off, and therefore display is possible immediately when the power source is next turned on.
A light emitting display device which includes active pixel circuits may also be used for displaying images both on its front-side and its back-side as well as in all possible directions of operation i.e. used as rectangular display in all vertical and horizontal positions. Thus each of the pixel circuits have to be operated by at least two different scan signals therefore being capable of performing a bi-directional scanning operation. The light emitting display includes a bi-directional signal transmission shift register, the pixel circuits, and a scan signal applier. Each of the pixel circuits has therefore to be provided with two or more scan lines. The shift register then outputs signals in their respective directions in response to related control signals. The scan signal applier sequentially applies, to the scan lines of the pixel circuits, scan signals corresponding to their respective direction control signals. The display device comprises further a bi-directional data driver including a bi-directional shift register to bi-directionally apply a data signal. That is, in an OLED display e.g. capable of implementing a double-sided display, images displayed on the front and back screens of the OLED display are horizontally inverted from each other (e.g., left to right and right to left). In order to display the same image on the front and back screens, accordingly, the order of applying data signals to the data lines in association with the image display on the front screen must be bi-directionally applied or reverse to the order of applying the data signals to the data lines in association with the image display on the back screen. For example, the last data signal to be applied to the last data line for the image display on the front screen must be applied to the first data line for the image display on the back screen. On the other hand, where it is desired to display the same image even when the display panel is inverted in the vertical direction as well as in the horizontal direction, for example, in accordance with a 180-degree rotation thereof (e.g., up to down and down to up or top to bottom and bottom to top), the scan driver must also use a bi-directional shift register to bi-directionally apply a scan signal, similar to the application of the data signal by the bi-directional data driver. That is, in the case of an OLED display including a 180-degree-rotatable display panel, a bi-directional scan driver is used to change the order of sequentially applying scan signals to scan lines between the sequential selection of the scan lines in a downward direction (forward scan) and the sequential selection of the scan lines in an upward direction (backward scan), and thus, to display the same image on the screen in both the non-rotated state and the rotated state. For example, the bi-directional scan driver applies the first scan signal, to be the first scan line in a forward scan mode, to the last scan line in a backward scan mode, and applies the last scan signal, to be applied to the last scan line in the forward scan mode, to the first scan line in the backward scan mode.
Problems and Solutions:
In the following some of the arising problems in conjunction with electro-optical displays and their proposed solutions are listed.
For typical pixels of the OLED display device of an active matrix system the light emission luminance of the OLED is controlled by an active device driving circuit constituted by TFT switch transistors and driver transistors, and an accumulation capacitor. More specifically, in its simplest case, the voltage corresponding to the electric charge which is accumulated in the accumulation capacitor through the switching transistors provides the gate voltage of a driver transistor, and the OLED is driven by the current which is determined on the basis of the gate voltage. However, in actual reality, there arises the problem that a display unevenness of the display picture is caused due to the non-uniformity of the threshold voltage and the charge drift mobility of the driver transistor.
An organic light emitting diode circuit used in organic light emitting display normally stores signals for controlling the luminance of an OLED via thin film transistors (TFTs) and capacitors. However, these TFTs, after prolonged use, will exhibit a threshold voltage shift. This shift amount is related to the operation time of the TFTs and the current flowing therethrough. In a display process, owing that the TFT for driving an OLED for each diode has a different current when turned on, and the driving TFTs will have different shift amounts of threshold voltage. As a result, the luminance of each diode will not have the same correspondence relation as the received pixel data, which in turn results in an uneven frame display.
In order to solve the above issue, voltage compensation techniques are applied to conventional OLEDs. In the above mentioned compensation technique, the compensation operation is performed during a data writing period to eliminate errors generated from the threshold voltage. However, recent OLED panels tend to be developed in high resolution and large size. As a result, the time for writing data is greatly reduced. However, the switching TFT has small current as it is turned on, and thus it needs longer compensation time, which will result in an irregular operation of the switching TFT and disability of the compensation mechanism. However, with the circuit design of a conventional voltage compensation arrangement, it is essential that the nodes connected to both X and Y line nodes have a stable voltage state during the “data writing” stage, otherwise, a charge sharing issue will be generated at the frame display stage. Accordingly, the conventional voltage compensation arrangement is apt to exhibit the drawback that the the node X does not reach a stable voltage state for canceling the threshold voltage of the driving TFT to be compensated due to inadequate time. In this situation, the switching TFT is still turned on. As a result, the charge sharing issue is generated and the display luminance can not reach the predicted luminance in correspondence with the pixel voltage. Voltage compensation techniques are thus used employing different levels of reference and supply voltages.
Another proposal is, that the magnitude of the current value of the writing current is temporally increased within the writing cycle, whereby the writing current is limited to a low level (or zero) in an early stage of the writing cycle. Thus, the average value of the writing current may be reduced. Additionally also a bias circuit for applying a bias voltage to the gate of the current supply transistor and thus producing a gate driver with improved reliability is proposed.
Display irregularities such as crosstalk, which develop due to a drop in voltage caused by the wiring resistance of electric current supply lines in an electro-optical device can also be avoided with active matrix technologies.
Display device solutions having a reduced data programming time are important and include a pixel display circuit which makes it possible to shorten a selection period per pixel while compensating variations in the threshold voltage of the driving transistor. The pixel circuit of the current programming method produces a long data programming time since it must charge and discharge parasitic capacitances generated at the data line. That is, the data programming time in the current programming type pixel circuit is influenced by the level of a voltage stored in the parasitic capacitance of the data line by the data current of the previous pixel line, and in particular, the data programming time is increased when the difference between the voltage of the data line and a target voltage (a voltage which corresponds to the current data) is large. In other words, the time for programming the data on the current pixel line is influenced by the voltage state of the data line according to the data of a previous pixel line, and in particular, the data programming time is further lengthened when the data line is charged with a voltage which has a large difference from the target voltage (the voltage corresponding to the current data). This phenomenon becomes the more important the lower the gray level (near black) or the higher the luminosity of color sub-pixels is.
Most voltage-control solutions allow the compensation of the threshold voltage fluctuations, but not also the compensation of fluctuations of the charge carrier movement. Additional TFT components for a current measuring- and voltage regulating circuit and a split conductor configuration are used which together are low-ohmic, so that in total very short response times are reached.
Another implementation of an active matrix display device has a monitoring portion which detects change of ambient temperature and degradation with time, provided with a plurality of monitoring pixels and a monitoring line. Each of the plurality of monitoring pixels has a light emitting element for monitoring, a constant current source, a switch, and a detecting circuit, and one electrode of the light emitting element for monitoring is connected to the monitoring line through the switch.
A technique, using a selection criteria for identification of any defective pixels in a matrix display during the initial screening and during operation has been established. According to this technique, the stability of the OLED can be checked by applying a reverse voltage over the OLED and detecting the resulting leakage current variation over time. Such a leakage current is small in the ideal device, but will be significantly larger if a defect is present. Therefore, defective pixels can be identified. On the contrary, in forward mode when the diode is ON, the current flowing through the diode is large, and any current contribution from a defect is hidden. The same effect can be utilized for using the pixel as a sensor. When subject to external influence, such as light, temperature, color, radiation or physical contact, the leakage current of the OLED will be altered. This alteration can be detected in the same way as mentioned above with regards to defects in the OLED. Techniques for correcting pixel defects have also been proposed for passive and active matrix displays. Strong voltage pulses are applied to an OLED in reverse mode. This high field can induce a high current to either heal or isolate a defect in a pixel.
Again another active matrix display device provides an image display that has an especially satisfactory display quality for animated images. A light emitting control switch means makes it can set a non-emission period of light between two consecutive frames by controlling a light-on time of the light emitting means in one frame. By setting an appropriate non-emission period of light, the afterimage effect that had appeared on the human visual property will lessen sufficiently within this non-emission period of light. Accordingly, the images for continuing two frames will not be superposed visually, which permits a smooth animated image display.
An active matrix OLED (AMOLED) display device may also make use of more sophisticated pixel and sub-pixel circuits whereby the scan driver includes a plurality of shift registers to output signals that are sequentially shifted, each of the shift registers controlling a start signal and more than two clock signals; and an output part to selectively output these clock signals and to switch between multiple supply voltages, which are different from each other, and which are switched for application to the pixel and sub-pixel circuits.
AMOLED displays may also comprise pixel electrode and display circuits capable of lowering power consumption with a uniformity of luminance retained, and realizing a display image having a high contrast and a high image quality, wherein additionally to the regular write and drive period for storing the written data signal and driving an OLED element in drive periods within the pixel control interval a correction period for correcting a variation in properties of internal drive transistors in a pixel circuit during a frame is used. Thus the pixel drive is controlled so that an interval having the correction period, the write period, and the drive period, and an interval having the write period and the drive period without the correction period exist.
This concludes the comparison of Active vs. Passive Matrix circuits.
A variety of solutions is found in the prior art for driving, controlling and addressing OLED displays in an attempt to simultaneously reach the two competing goals namely reaching high accuracy for OLED displays' brightness control and low power consumption and effectiveness in continuous operation. Nevertheless, additional improvements in both fields are desired and continued improvements in these areas are needed. It is therefore a challenge for the designer of such circuits to achieve an even higher accuracy in OLED pixel brightness control and also a more power economical solution which is also furnishing a better life-time. There are various patents referring to such solutions.
WIPO Patent Application (WO/2007/079947 to Xu et al.) teaches a method for triggering matrix displays, wherein matrix displays (D) are described which are composed of several rows that comprise individual pixels (ij) and are configured as lines (i) and columns (j). In said method, individual rows are selectively triggered by activating lines (i) for a certain line addressing time (ti), and the columns (j) are impinged upon by an operating current (l) or a corresponding voltage in correlation with the activated line (i) according to the desired brightness (Dij) in the pixels (ij). In order to increase the performance of the display, the line addressing time (ti) for each line (i) is defined in accordance with the maximum brightness of all columns (D).
U.S. Patent Application (2007/046603 to Smith et al.) provides multi-line addressing methods and apparatus, whereby this invention relates to methods and apparatus for driving electroluminescent, in particular organic light emitting diodes (OLED) displays using multi-ling addressing (MLA) techniques. Embodiments of the invention are particularly suitable for use with so-called passive matrix OLED displays. A current generator for an electroluminescent display driver, the current generator comprising: a first, reference current input to receive a reference current; a second, ratioed current input to receive a ratioed current; a first ratio control input to receive a first control signal input; a controllable current mirror having a control input coupled to said first ratio control input, a current input coupled to said reference current input, and an output coupled to said ratioed current input; said current generator being configured such that a signal on said control input controls a ratio of said ratioed current to said reference current.
U.S. Patent Application (2008/0246703 to Smith et al.) discloses display driving methods and apparatus for driving a passive matrix multicolor electroluminescent display, wherein a method of driving a passive matrix multicolor electroluminescent display is teached, the display comprising a plurality of pixels arranged in rows and columns, each said pixel comprising at least first and second sub-pixels having different respective first and second colors, the method comprising: driving groups of said pixels in turn to display a multicolor image frame, said driving of a group of pixels comprising driving first and second sub-groups of sub-pixels of respective said first and second colors; and wherein said driving further comprises driving a said group of pixels for a duration dependent upon a maximum drive level of a sub-pixel of a said sub-group.
U.S. Patent Application ((2008/0211793 to Ku, Chiung-Ching) discloses a driving apparatus for an OLED panel which comprises a common timing controller with a multi-line addressing algorithm and a segment controller with the multi-line addressing algorithm, to control a common driver and a segment driver, respectively, to drive the OLED panel. The common driver and the segment driver are connected to common lines and segment lines of the OLED panel, respectively. According to the multi-line addressing algorithm, the common driver can select at least two of the common lines at a same time.
Studying both MLA methods mentioned above, TMA and CMLA, they are disclosing substantial technically and mathematically different ways of working. TMA is based on a non-negative matrix factorization using an iterative method to approximate the original image by a product of two non-negative matrices, and TMA therefore involves complex processing of the whole image along with driver circuits using well matched current sources and sinks for both anodes and cathodes of the OLEDs whereas CMLA tries a lossless decomposition of the image data matrix into a sum of consecutive multi line matrices by a combinatorial algorithm, i.e. computed only by adding, substracting and comparing operations also using sequences of iterations, whereby at least three are needed, although simpler to implement than TMA it does nevertheless require significant processing of the image in order to minimize the peak currents for the OLED drivers.
In the prior art, there are different technical approaches to achieve the goal of implementing modern MLA schemes for FPD driving with limited processing power requirements in favor of a low power consumption and augmented life-time realized as integrated circuits. However these approaches use often solutions, which are somewhat technically complex and therefore also expensive in production. It would therefore be advantageous to reduce the expenses in both areas.